The invention relates to a machine tool in which a bed-slide movable on a guide-way provided on the bed is fed by a motor. A pair of racks is mounted in fixed relationship to the bed-slide on either side of the guide-way. Pinions meshing with the racks are journalled on the bed-slide and are connected with the feed motor by a transmission. An example of such a machine tool is a boring machine in which the bed-slide carries a horizontal boring tool fixed thereto to be fed into a rotary workpiece carried by a rotary spindle journalled in a spindle housing mounted on the bed.
The provision of a pair of racks results in a symmetrical structure of the bed-slide with respect to the vertical longitudinal central plane, in contrast to such types of machine tools in which a single rack is fixed to the bed-slide laterally. The symmetry of the structure with respect to said longitudinal plane will avoid any interference with the feed motion of the bed-slide of oblique forces liable to produce undesired clamping effects where the driving force is introduced into the bed-slide in a non-symmetrical manner. Such non-symmetrical exertion of the driving force, however, may also occur in spite of the provision of a pair of racks, where the pinions meshing with the racks are driven with different driving torques owing to inaccuracies of the transmission connecting the feed motor with the pinions.
It is the object of the present invention to avoid such imbalance of the driving torques exerted on the pinions and to insure an equal distribution of the feeding force on both racks.
This object is attained according to the present invention by the provision of a differential transmission having an input connected with the feed motor and two outputs, each output being connected with one of the pinions.
While this differential transmission could be designed in a manner similar to that of a motor vehicle connecting the driving motor with the driven wheels of the vehicle, it is another object of the invention to simplify the differential transmission considerably compared with the conventional differential transmissions of motor vehicles. This object is attained, according to another aspect of the invention, by such a design of the feeding mechanism that a pair of gear sets each acting on one of the racks have a common input shaft driven by the motor and helical gears of opposite hand fixed to said shaft for common displacement therewith. Any tendency of this transmission to imbalance the driving torques exerted on the two sets of gears results in an axial displacement of the shaft and in a consequent relative actuation of the two sets of gears in the opposite sense of rotation to thereby balance the driving torques.
Where the invention is applied to a machine tool in which the bed-slide is movable on top of the bed, the pinions engaging the racks and driven by the differential transmission are preferably mounted below the racks. This will prevent the bed-slide from being lifted from the bed as might occur where the bed-slide is fed by a considerable force contrary to a counter-force acting on the bed-slide above the bed. These and other features of the present invention will be discussed in greater detail below with reference to the drawings accompanying this specification.